A membrane can be described as a thin barrier between the two bulk phases and it is either a homogeneous phase or a heterogeneous collection of phases. The membrane is a permselective barrier that permits transport of some component but retains others. The flow of material across a membrane is kinetically driven by the application of pressure concentration, vapor pressure, hydrostatic pressure, electrical potential, or temperature.
In membrane separations, each membrane has the ability to transport one component more readily than the other because of differences in physical and chemical properties between the membrane and the permeating components. Furthermore, some components can freely permeate through the membrane, while others will be retained. The stream containing the components that permeate through the membrane is called permeate and the stream containing the retained components is called retentate. The transport of permeate across the membrane is achieved by the application of either mechanical, chemical, electrical or thermal works.
Membrane separation process has emerged as a separation technology that undergoes a rapid growth during the past few decades. It has drawn the world attention especially in the separation technology field, which is competitive in many ways with conventional separation techniques such as distillation, adsorption, absorption and extraction.
Practically all filtration membrane is based on synthetic organic (polymeric or liquid) or inorganic (ceramic, metal) membrane. But for the sake of simplicity only the synthetic organic polymer membranes are discussed here. A synthetic organic polymer can be classified according to different viewpoints such as morphology or separation mechanism.
In term of morphology, two types of membrane were distinguished: symmetric or asymmetric. In the symmetric membranes, the diameter of the pores is almost constant throughout the cross section of the membrane. Furthermore, the entire membrane thickness causes resistance to mass transfer acting as a selective barrier. In asymmetric membrane, the pore size at the surface have a different size compared with the pores at the bottom side. Large particles will not enter in the body of the membrane. In this way, the plugging of the membrane is avoided. It also possible that the top layer is non-porous or even made of a different material and such membrane is called composite membrane. The composite membrane composed of two layers, a support layer and a skin layer. The support or porous layer has high porosity, no selectivity and a thickness of 50 to 150 mm. The skin or top layer is very thin (0.1 to 5 mm) and it is responsible for the membrane selectivity. The actual separation mechanism can be based on following:
i. Separation by size – the sieve effect.
This requires porous membrane with rather large pores. These mechanisms are the simplest form regarding mode of separation. Terms like macropores, mesopores and micropores are used to describe the pore sizes in membrane for microfiltration, ultrafiltration and nanofiltration. These membranes have been designed to retain larger solute and suspended particles thus remove contaminants based on size by sieving mechanism.